The present invention pertains to a radiation-curable inkjet ink and, more particularly, to a radiation-curable inkjet ink comprising a specified polymerizable vehicle that is curable on exposure to radiation such as ultraviolet light.
Inkjet imaging techniques have become very popular in commercial and consumer applications. Ink jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can produce a wide variety of printed features, including text, graphics, images, holograms and the like. Moreover, ink jet printers are capable of forming printed features on a wide variety of substrates, as well as three-dimensional objects in applications such as rapid prototyping.
Inkjet inks must meet stringent performance requirements in order for the inks to be appropriately jettable and for the resultant printed features to have the desired mechanical, chemical, visual and durability characteristics. In particular, inks must have relatively low viscosity when jetted, yet must be able to form accurate, durable images on the desired receiving substrate. For example, a typical ink will have a viscosity in the range of 3 to 30 centipoise at the jetting temperature. The low viscosity, however, poses a substantial challenge to achieving printed features with good mechanical and durability characteristics.
Aqueous based inks, although very successful in the home and small office market, have certain drawbacks in other applications. For example, in industrial printing, the substrate is typically nonporous and the aqueous ink must be dried, which is equipment intensive and time consuming. Also, the printed material needs to be handled carefully during the relatively lengthy drying period. Water-based inks are also compatible only with a limited range of substrates, and images formed using water-based inks typically require a protective overlaminate for outdoor applications.
Solvent-based inks, which are most commonly used in industrial applications, contain relatively volatile organic solvents. These inks dry more readily than aqueous inks and tend to be somewhat more durable in outdoor applications. However, the solvents require careful handling and may be toxic and/or flammable. These inks also tend to be compatible with only a limited range of substrates.
To avoid using conventional solvents, inks with polymerizable diluent have been developed. The diluent generally comprises one or more reactive monomers that are polymerized by exposure to radiation (radiation curable) such as ultraviolet light, electron beam energy and the like. The cured diluent forms a polymer film that provides durability to the print and requires no drying. The diluent mixture, which is also the ink vehicle, is chosen to provide appropriate ink viscosity. The reactive monomer content in the diluent also impacts the physical properties of the printed image (durability, flexibility, elasticity, gloss, hardness, chemical resistance, stiffness and so forth) and is optimized to achieve the desired properties.
Many of the polymerizable monomers have problems with regard to skin and/or eye irritation, and many of the monomers having low irritation cause undesirable cured properties such as poor weatherability in outdoor applications. Preferably, curable inks are comprised of polymerizable monomers with low irritation and, even more preferably, also provide good as-cured properties.
Atmospheric oxygen can inhibit polymerization of curable inks, reducing the quality and speed of curing. Oxygen inhibition can be avoided by curing materials in an inert atmosphere, but this is generally impractical. It is desirable to provide ink jet inks that cure rapidly in ambient conditions with little or no sensitivity to atmospheric oxygen.
Inkjet inks must possess proper dot gain in order to form images with attractive appearance. Dot gain refers to the degree to which an ink jetted drop spreads out upon application to a substrate. If an ink jetted drop spreads out too much on the substrate, then poor edge definition and intercolor bleed is observed. On the other hand, if an ink jetted drop spreads insufficiently upon application to the substrate, poor color density results. Dot gain characteristics depend upon a number of factors including the ink jet composition, the nature of the substrate, the substrate temperature, and the interfacial tension between the ink and the substrate. Many of the currently available radiation curable inks show favorable dot gain characteristics on some substrates, but not on others. Preferably a curable inkjet ink would be formulated to provide good dot gain characteristics and good appearance on a broad range of substrates.
Radiation curable inkjet inks with polymerizable diluent are disclosed, for example, in WO02/061001, WO05/030881, U.S. Pat. No. 6,685,311 and U.S. Pat. No. 6,913,352.
Radiation curable inkjet inks with a polymerizable silicone derivative as part of the polymerizable diluent are disclosed, for example, in U.S. Pat. No. 6,593,390 and WO05/047405. The polymerizable silicone derivative, such as silicone acrylate, can modify the surface tension of the ink.
An ink-jettable, radiation-curable overprint composition is disclosed in US2005/0249895. The composition comprises a polymerizable diluent and a surfactant, such as a polyether modified polydimethylsiloxane or a fluorosurfactant, to reduce the surface tension.
A radiation curable inkjet ink free of unreactive volatile organic compounds comprising polymerizable diluent and poly(alkylene oxide) modified poly(dimethyl siloxane) surfactant is disclosed in WO05/030879.
All of the above-referenced disclosures are incorporated by reference herein for all purposes as if fully set forth.
It is, therefore, desirable to provide radiation curable inks with improved characteristics, such as image durability and/or cure speed, and that are also suitable for use on a broad range of substrate types.